As shown in the configuration diagram of FIG. 14, a conventional elastic wave filter of this type has unbalanced signal terminal 16; and first, second, and third interdigital transducer (IDT) electrodes 17A, 17B, and 17C whose wiring electrodes are electrically connected to unbalanced signal terminal 16. The conventional elastic wave filter also has fourth IDT electrode 17D disposed between first and second IDT electrodes 17A and 17B, and fifth IDT electrode 17E disposed between second and third IDT electrodes 17B and 17C. The conventional elastic wave filter also has first balanced signal terminal 18A electrically connected to the wiring electrode of fourth IDT electrode 17D, and second balanced signal terminal 18B electrically connected to the wiring electrode of fifth IDT electrode 17E. In the conventional elastic wave filter, the ground electrodes of first, second, third, fourth, and fifth IDT electrodes 17A, 17B, 17C, 17D, and 17E are electrically connected to the ground. In the conventional elastic wave filter, a signal in opposite phase with the signal input from unbalanced signal terminal 16 is output from first balanced signal terminal 18A, and a signal in phase with the signal input from unbalanced signal terminal 16 is output from second balanced signal terminal 18B. In the conventional elastic wave filter, the ground electrodes of second and third IDT electrodes 17B and 17C are adjacent to the wiring electrode of fifth IDT electrode 17E, and the ground electrodes of first and second IDT electrodes 17A and 17B are adjacent to the ground electrode of fourth IDT electrode 17D (see Patent Literature 1, for example).
However, in such a conventional elastic wave filter, spuriousness occurs in the bandpass.
That is, in the above conventional configuration, the ground electrodes of first and second IDT electrodes 17A and 17B are adjacent to the ground electrode of fourth IDT electrode 17D, and thus spuriousness “S” occurs in the bandpass, as shown in the bandpass characteristics of FIG. 15.
Further, an elastic wave filter where the bandpass characteristics are improved by parallel-connecting the elastic wave filters of FIG. 14 is known. As shown in FIG. 16, this conventional parallel-connected elastic wave filter has first longitudinally coupled resonator elastic wave filter 5 and second longitudinally coupled resonator elastic wave filter 9 formed on piezoelectric substrate 1. The first longitudinally coupled resonator elastic wave filter has first unbalanced signal terminal 2, and first balanced signal terminal 3 and second balanced signal terminal 4 electrically connected to first unbalanced signal terminal 2. The second longitudinally coupled resonator elastic wave filter has second unbalanced signal terminal 6, and third balanced signal terminal 7 and fourth balanced signal terminal 8 electrically connected to second unbalanced signal terminal 6.
First unbalanced signal terminal 2 and second unbalanced signal terminal 6 are electrically connected to each other. First balanced signal terminal 3 and second balanced signal terminal 4 are electrically connected to first input/output terminal 10. Third balanced signal terminal 7 and fourth balanced signal terminal 8 are electrically connected to second input/output terminal 11. The input/output signal from first input/output terminal 10 is 180° out of phase with the input/output signal from second input/output signal 11 (see Patent Literature 2, for example).
However, in such a conventional parallel-connected elastic wave filter, the insertion loss degradation is large in the high-frequency part of the band.
FIG. 17 is a chart showing bandpass characteristics of a conventional parallel-connected elastic wave filter. That is, in the conventional parallel-connected elastic wave filter, as shown in FIG. 17, spuriousness “S” occurs in the high-frequency part of desired bandpass PB0. As a result, the insertion loss degradation is increased.
[Patent Literature 1]
Japanese Patent Unexamined Publication No. 2001-313540
[Patent Literature 2]
Japanese Patent Unexamined Publication No. 2002-314371